Enthalpy Calculator — ΔH for Reactions Using Hess's Law and Formation Data
Calculate enthalpy change (ΔH) for chemical reactions using standard enthalpies of formation or Hess's Law. Find heat released or absorbed in any reaction.
Enthalpy change tells you how much heat a reaction releases or absorbs at constant pressure — which covers nearly every reaction that happens in a flask, a body, or an engine. A negative ΔH means heat flows out (exothermic); a positive ΔH means heat flows in (endothermic). The CalcHub Enthalpy Calculator computes ΔH using standard formation enthalpies and Hess's Law so you don't need to look up and combine values by hand.
The Core Formula
ΔH°rxn = Σ ΔHf°(products) − Σ ΔHf°(reactants)
Where ΔHf° is the standard enthalpy of formation (enthalpy change when 1 mol of compound forms from its elements in standard states).
By definition, ΔHf° of any element in its standard state = 0 (e.g., O₂(g), Fe(s), C(graphite)).
How to Use the Calculator
- Open CalcHub and go to the Enthalpy Calculator.
- Enter the balanced chemical equation.
- The tool looks up standard formation enthalpies from its database, or you can enter custom values.
- It applies the formula with stoichiometric coefficients as multipliers and returns ΔH°rxn.
Worked Example
Combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)| Substance | ΔHf° (kJ/mol) | Coefficient | Contribution |
|---|---|---|---|
| CO₂(g) | −393.5 | 1 | −393.5 |
| H₂O(l) | −285.8 | 2 | −571.6 |
| CH₄(g) | −74.8 | 1 | −74.8 |
| O₂(g) | 0 | 2 | 0 |
Highly exothermic — which is why natural gas makes an effective fuel.
Hess's Law
Hess's Law states that enthalpy is a state function: the total enthalpy change is the same regardless of the pathway taken. This lets you combine multiple reactions algebraically:
- If you reverse a reaction, flip the sign of ΔH
- If you multiply a reaction by n, multiply ΔH by n
- Add the equations and their ΔH values
Exo vs. Endothermic
| ΔH | Type | Heat direction | Examples |
|---|---|---|---|
| Negative | Exothermic | Released to surroundings | Combustion, neutralization |
| Positive | Endothermic | Absorbed from surroundings | Photosynthesis, dissolving NH₄NO₃ |
What's the difference between ΔH and ΔU (internal energy)?
ΔH = ΔU + PΔV. For reactions at constant pressure, ΔH accounts for work done by or on the gas phase. For reactions involving only solids and liquids (negligible PΔV), ΔH ≈ ΔU. For gas-phase reactions with significant molar volume changes, they differ.
Why is ΔHf° = 0 for elements in standard state?
Formation enthalpy is defined relative to elements in standard state. If you start with an element in standard state and "form" it again, no change occurs, so no enthalpy change. It's the reference baseline, not a measurement.
How do I find ΔH if I don't have all the formation data?
Use Hess's Law by combining reactions whose enthalpies you do know. This is often how textbook problems are structured — giving you three equations to combine into the one you need. The calculator's Hess mode automates this combination.
Related calculators: Rate of Reaction Calculator · Electrochemistry Calculator · Stoichiometry Calculator